Antonoff co-wrote Swift's song, "Sweeter Than Fiction," which will appear or the upcoming "One Chance" soundtrack; the film is about "Britain's Got Talent" winner Paul Potts.

"She's just like the most incredible writer and singer and performer," Antonoff said in a recent interview. "She feels completely unaffected by all the stuff happening around her, so working with her was like the most amazing experience. ... It made me better at what I do just watching her process."

Antonoff, 29, added that he and the country-pop princess decided to work together after he told her about a song he was writing that featured the snare drum. He added that the collaboration worked so well because there was "no label involved, no one put us together."

Fun. is still riding high — and on tour — off the success of its platinum-selling sophomore album, which launched the hits "Some Nights" and "We Are Young" last year. The latter track won the Grammy for record of the year and helped the trio win best new artist. Antonoff co-wrote the entire album with band mates Nate Ruess and Andrew Dost. He said songwriting outside of the band is helping the group.

"I'm doing a lot of other stuff with other people. I did some stuff with Christina Perri and I'm working with some other people that I'm not allowed to say yet," he said. "The more I can do that the more I kind of constantly re-fall in love with all the elements of what I do, so I think it's really good for me, it's good for my band."

Antonoff also co-wrote Sara Bareilles' "Brave," which became a Top 40 hit after music fans said Katy Perry's "Roar" sounded like the piano tune.

"I hear what everyone else hears," Antonoff said with a laugh of the comparisons. "I hear the similarities. But at the end of the day, you know, I think Katy Perry releasing 'Roar' put a big spotlight on 'Brave' also, so if we can find ourselves in a situation where two great songs are present in the public then that's great."

Bareilles recently said she doesn't feel like Perry ripped off her song.

"I don't think the songs sound the same, but I think there's a lot in their production that feels similar," Antonoff added. "It's cool if Katy Perry was anyway inspired by 'Brave' — that's awesome."

Fun. performed Friday as part of the iHeartRadio music festival in Las Vegas. The group sang its own hits and then joined Queen onstage, along with Adam Lambert.

"I wouldn't play guitar if there was no Brian May. We wouldn't be a band if it wasn't for Queen," Antonoff said. "It was one of the most incredible experiences."

Page 2 of 2 - The pop-rock group's "Most Nights" tour wraps Oct. 13 in Mexico City at the Corona Capital Festival. Antonoff said they'll likely head back in the studio to record the follow-up to "Some Nights" in March and he's happy with how his band has dealt with their massive success.

"I'm still alive, I'm still in a relationship, I haven't (messed) everything up yet, but it's not easy," said Antonoff, who is dating "Girls" actress and creator Lena Dunham. "It's tough to be away — it's that simple. You miss things. A year ago it was real (messed) up, it was hard. There was no way you can learn how to be in that schedule and be under those kinds of pressures."

"I think that we went through a lot and I think we got through it," he said of group. "And none of us turned to drugs or none of us ended as shady people."